1. Field of the Invention
The present invention relates to a method of fabricating semiconductor integrated circuits, and more particularly, to a method of fabricating MMICs (Monolithic Microwave Integrated Circuits) which can be made compact.
2. Description of the Prior Art
In microwave integrated circuits, hybrid integrated circuit techniques for mounting an active device and a passive circuit device on a dielectric substrate have been heretofore used. In recent years, however, MMICs suitable for compactness and quantity production have been put into practice. MMICs using an HEMT (High-Electron-Mobility Transistor), a GaAs MES FET (GaAs Metal-Semiconductor Field Effect Transistor), an HBT (Hetero-Bipolar Transistor) or the like are more favorable for compactness and light weight, as compared with hybrid ICs such as MICs (Microwave Integrated Circuits). In addition, the MMICs are superior in quantity production and reliability. Consequently, the MMICs are expected as key devices of various microwave equipments.
Circuit elements of an MMIC include an active device such as an HEMT, a GaAs MES FET or an HBT and a passive circuit device such as a transmission line (a microstrip line, a coplanar line or the like), a capacitor, an inductor or a coupler. As a method of forming the devices, microwave design techniques are required so as to improve matching of both the devices.
As a method of microwave matching, the passive circuit device is generally matched in characteristics with the active device by simulation using a concentrated constant or distributed constant circuit. A circuit pattern of a passitve circuit device has been conventionally determined as the first meaning by simulation based on the average value of DC and/or RF characteristics of several active devices formed on the substrate or by simulation based on a target value of the DC and/or RF characteristics. A photomask or the like is previously made on the basis of this circuit pattern determined. In actually fabricating an integrated circuit, an active device is first formed on a substrate and then, a passive circuit device is formed using this photomask. In general, if active devices are respectively formed on substrates under the same forming conditions, characteristics of the active devices differ depending on the substrates due to the variation in forming process. On the other hand, even if active devices are formed in the same substrate, characteristics of the active devices differ depending on the positions where they are formed. Consequently, in the conventional method of matching using a uniform photomask pattern, the active device and the passive circuit device may not be matched with each other. In such a case, mismatching of both the devices does not become clear until the time point when the MMIC is actually fabricated, resulting in a problem of low fabrication yield.
Additionally, days are required to make the photomask, thereby encountering a problem of requiring too much time until a single circuit characteristic can be measured.